Electrical circuits requiring high power handling capability (>20 watts) while operating at high frequencies such as radio frequencies (500 MHz), S-band (3 GHz) and X-band (10 GHz) have become commonplace. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors that are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads. Metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction may be preferable for high frequency applications because only majority carriers carry current. The MESFET design may be preferred over other designs because the reduced gate capacitance may permit faster switching times of the gate input. Therefore, although generally all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET may make the MESFET more desirable for high frequency applications.
Silicon carbide (SiC) has been known for many years to have excellent physical and electronic properties which should theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon (Si) or GaAs. The high electric breakdown field of about 4×106 V/cm, high saturated electron drift velocity of about 2.0×107 cm/sec and high thermal conductivity of about 4.9 W/cm-° K indicate that SiC would be suitable for high frequency, high power applications.
SiC MESFETs fabricated on high resistivity substrates have found widespread use for high power RF amplifiers. Devices having highly doped p-type layers under the source region of the FET have been provided and have been successful in providing high breakdown voltages for power amplifiers, while reducing drifts in device characteristics arising from trapping in semi-insulating substrates. These devices are discussed, for example, in commonly assigned U.S. Pat. No. 6,956,239 to Sriram.